Optical fiber transmission systems often require that the signal carried through the system be reduced or attenuated before it can be used. Such attenuation generally is carried out using either an attenuator that can be adjusted in situ to provide a desired attenuation level, or in the alternative, by using an attenuator having a fixed, predetermined attenuation factor that is selected prior to use for the specific application. The latter type of attenuator is referred to herein as a "fixed value fiber optic attenuator". The majority of the fixed value attenuators currently used in fiber optic transmission systems rely upon passive filters, air gaps, optical wedges and/or lenses to reduce the signal strength in optical transmission lines.
With increasing data transmission speeds, a phenomenon referred to as "back-reflection" or "return loss" has been identified as causing significant losses in signal strength and quality. The phenomenon is characterized by Fresnel reflections which occur when light is transmitted from a first region having one index of refraction to a second region having a different index of refraction.
Current optical fiber attenuators generally operate by employing materials having refraction indices that differ from the refraction index of the optical transmission fiber to thereby cause a reduction in optical signal strength. Thus, back-reflection, and its resultant signal deterioration, is an inherent characteristic of most attenuation systems currently in use.
In addition, most of the attenuators used in fiber optic transmission systems rely upon materials that provide a differing degree of attenuation for differing wavelengths of light. Thus, it becomes necessary to identify the wavelengths to be utilized in the transmission system prior to selecting the specific attenuators. Otherwise, if the wavelengths carried through the system vary, the attenuation will likely vary as well.